The known technologies available for debris detection in the airport environment have been examined and areas for improvement identified. It has been found that a novel mobile sensing technique is highly appropriate for this application in the long term.
Much of the following discussion focuses upon runway applications. It is, however, noted that the present invention may also be deployed in other locations where surface obstruction or contamination may be a problem. Embodiments of the present invention may also be applied to the detection of undesired objects on roads and major highways; in shipping lanes and at the dock-side; and even in factories. As will be apparent, the consequences of the presence of undesired objects on airport runways can be exceptionally severe thus there is a particular need to address runway applications.
Undesired object damage due to debris on the pavement can occur anywhere in the aircraft maneuvering or movement areas of an airport. Damage can be caused by jet blast from one aircraft blowing debris onto another, or onto people, or onto logistics and infrastructure items. Damage can also occur directly to an aircraft striking the debris.
A major event in which an aircraft sustains significant damage can cause delay or cancellation of its flight, with the consequent knock-on effects of re-scheduling, increased workloads for airport and airline personnel, and loss of customer satisfaction. The major costs will be those of repairs and third-party liability claims. If debris is ingested by a jet engine, repair costs can rise to millions of Euro.
On an active runway, the consequence of a undesired object damage event can be more severe due to the higher kinetic energies involved; it can even lead to loss of the aircraft, and consequent loss of life. Take-off accidents are likely to be particularly severe due to the large fuel load. Such an event is likely to result in closure of the airport for a significant period, causing knock-on effects throughout the Air Navigation System of the region, which may increase the likelihood of further accidents elsewhere.
There are documented examples of catastrophic aircraft losses due to both non-metallic and metallic objects on a runway.
Debris on the runway can come from jet blast effects; for example, an aircraft turning from the active runway onto a narrow taxiway can blow material from the shoulders and in-field areas backwards on to the runway. Items may also fall from the aircraft themselves, or other vehicles, causing a hazard for subsequent users of the runway. In addition, other undesired objects need to be detected on the runway and in other active areas: examples of these undesired objects include misplaced tools, rubbish and even animals.
The current approach to this problem is for someone to physically go out onto the runway and look. This is usually done in a vehicle such as a Land Rover, which has to be in radio contact with the ground and air traffic controllers to co-ordinate runway occupancy with aircraft movements. At a busy airport, the debris monitoring operation will tend to be continuously interrupted by aircraft operations; it may even constitute a constraint to runway capacity.
It is apparent therefore, that a need exists for an automated debris sensor system.
Where it is known to use a radiometer to detect vehicles against a road surface (see UK patent application, Pub. No. GB 2358269), recent work has shown that it would also be practicable to employ radiometers to detect recently deposited debris that is out of thermal equilibrium with the runway. Particularly good results will also be obtained from metallic objects that reflect the sky temperature to the sensor.
The use of millimeter wave radar is also known for the detection of small objects on the ground, but the results are severely limited due to the clutter returns, it has been found that better results are obtained when synthetic aperture radar processing is employed. Synthetic aperture radar using the focused processing method can be mounted on a rail adjacent to the area to be monitored and be arranged to run backwards and forwards in a controlled manner. Alternatively, unfocussed synthetic aperture processing can be employed to give reasonable results from a sensor mounted on a vehicle, whose motion is less predictable.
Sensors all have their strengths and weaknesses in this application; a camera system, for example, may interpret a skid mark on the runway as a piece of flat debris, radar is unlikely to see it at all. Conversely, a radar system may get a strong signal echo from a small piece of metal foil that would not be a hazard, and which a camera would not see. Combining the data from diverse sensors will give a better overall result and reduce false alarms.
Three main strategies for deployment of sensors can be adopted. In a first strategy, a high number of fixed installations can be used in order to get short-range coverage of the entire surface of interest; in a second, a mobile system can be used, the system mirroring that being used at present, viz. inspection Land Rovers, in this way the sensors are moved close to the observation areas over time; and finally these fixed and mobile systems can be combined, whereby the fixed sensors could continuously monitor the regions of highest risk while the mobile system can patrol the remaining active areas.
The current inspection vehicles operate on busy runways and can take up to forty-five minutes per sweep; these sweeps are carried out relatively infrequently during the day. It is clear that the mobile system carrying sensors should be more vigilant and provide higher coverage rates than the existing approach.